The army doesn't necessairly HAVE to follow the social values and customs of the nation it defends, it just usually does because it's made up of the members of that nation. If we had an all merc/PMC army then they probably wouldn't be close to our values structure simply because they aren't in it to defend their lands, they're in it for cash.
However, the military is usually the first or last part of the society to change, simply because of the nature of how one joins and then proceeds into a position of power within it's structure. If there could be a popular demonstration and walk out by the new soldiers over these sorts of issues it'd probably be faster, but *shrugs*